Someone at Yale University should have dressed up as Robespierre for Halloween, as its students seem to have lost their minds over what constitutes a culturally appropriate costume. Identity and grievance politics keeps hitting new lows on campus, and now even liberal professors are being consumed by the revolution.

On Oct. 28 Yale Dean Burgwell Howard and Yale’s Intercultural Affairs Committee blasted out an email advising students against “culturally unaware” Halloween costumes, with self-help questions such as: “If this costume is meant to be historical, does it further misinformation or historical and cultural inaccuracies?” Watch out for insensitivity toward “religious beliefs, Native American/Indigenous people, Socio-economic strata, Asians, Hispanic/Latino, Women, Muslims, etc.” In short, everyone.

Who knew Yale still employed anyone willing to doubt the costume wardens? But in response to the dean’s email, lecturer in early childhood education Erika Christakis mused to the student residential community she oversees with her husband, Nicholas, a Yale sociologist and physician: “I don’t wish to trivialize genuine concerns,” but she wondered if colleges had morphed into “places of censure and prohibition.”

And: “Nicholas says, if you don’t like a costume someone is wearing, look away, or tell them you are offended. Talk to each other. Free speech and the ability to tolerate offence are the hallmarks of a free and open society.”

Some 750 Yale students, faculty, alumni and others signed a letter saying Ms. Christakis’s “jarring” email served to “further degrade marginalized people,” as though someone with a Yale degree could be marginalized in America. Students culturally appropriated a Puritan shaming trial and encircled Mr. Christakis on a lawn, cursing and heckling him to quit. “I stand behind free speech,” he told the mob.

Hundreds of protesters also turned on Jonathan Holloway, Yale’s black dean, demanding to know why the school hadn’t addressed allegations that a black woman had been kept out of a fraternity party. Fragile scholars also melted down over a visiting speaker who made a joke about Yale’s fracas while talking at a conference sponsored by the school’s William F. Buckley, Jr. program focused on . . . the future of free speech.

The episode reminds us of when Yale alumnus Lee Bass in 1995 asked the university to return his $20 million donation. Mr. Bass had hoped to seed a curriculum in Western civilization, but Yale’s faculty ripped the idea as white imperialism, and he requested a refund. Two decades later the alternative to Western civilization is on display, and it seems to be censorship.

According to a student reporting for the Washington Post, Yale president Peter Salovey told minority students in response to the episode that “we failed you.” That’s true, though not how he means it. The failure is that elite colleges are turning out ostensible leaders who seem to have no idea why America’s Founders risked extreme discomfort—that is, death—for the right to speak freely.